In general, an end facet reflective film is provided on an end facet of the laser light resonant cavity of a semiconductor laser device. Particularly, a reflective end facet, which is a rear end facet opposite to the laser-light-emitting facet, should have a high reflectance. Thus, an end facet reflective film with a high reflectance is formed by alternately stacking low- and high-refractive-index films having thicknesses of λ/4n1 and λ/4n2, respectively, where λ is an oscillation wavelength of the laser light, n1 is a refractive index at the low-refractive-index film at the wavelength of λ, and n2 is a refractive index of the high-refractive-index film at the wavelength of λ.
Each of the low- and high-refractive-index films making up the end facet reflective film should have a sufficiently small absorption coefficient at the wavelength of the laser light. Thus, silicon dioxide (SiO2) or aluminum oxide (Al2O3), each of which has a small absorption coefficient in a wide range covering visible to ultraviolet light parts of the spectrum, is used for the low-refractive-index films in the end facet reflective film. On the other hand, various dielectric materials are usable for the high-refractive-index film in the end facet reflective film depending on the wavelength of the laser light.
For example, in an infrared or red-light-emitting semiconductor laser device of aluminum gallium arsenide (AlGaAs) that outputs a laser beam with a wavelength of about 780 nm, amorphous silicon (α-Si) is used for the high-refractive-index films of the device. In this case, the absorption coefficient of amorphous silicon to light with a wavelength of 780 nm is 4×104 cm−1.
Exemplary applications of this infrared or red-light-emitting semiconductor laser device to the field of optical disk apparatuses include a laser device for a 4×CD-R (CD-recordable), on which data can be written only once four times faster than normal speed. In the laser device for the 4×CD-R, a multiple pairs of silicon dioxide and amorphous silicon films are stacked as an end facet reflective film for the rear end facet. For example, if the end facet reflective film is made up of two pairs (cycles) of silicon dioxide and amorphous silicon films, the reflectance can be 95%.
By using this end facet reflective film, a laser device for a 4×CD-R can have an optical output of 100 mw in a pulsed drive mode at a duty cycle of 50% or 80 mW in a continuous-wave (CW) drive mode.
On the other hand, titanium oxide (TiO2) is used instead of amorphous silicon for the high-refractive-index films of a red-light-emitting semiconductor laser device of aluminum gallium indium phosphide (AlGaInP) that outputs a laser beam with a wavelength of about 650 nm. Amorphous silicon is not used because of the following reason. If amorphous silicon was used for the end facet reflective film, light absorbed into the amorphous silicon layer would increase because the absorption coefficient of amorphous silicon to light with a wavelength of around 650 nm is high. Due to a rise in temperature caused by this light absorption, the crystallinity of the laser device in the vicinity of the resonant cavity end facet thereof deteriorates, and thus the reliability of the device declines.
In view of this, titanium oxide, having a refractive index sufficiently higher than that of silicon dioxide and an absorption coefficient lower than that of amorphous silicon, is used for the end facet reflective film of the red-light-emitting semiconductor laser device with a wavelength of about 650 nm. The absorption coefficient of amorphous silicon to light with a wavelength of 650 nm is 1×105 cm−1, while the absorption coefficient of titanium oxide to light with a wavelength of 650 nm is 2 cm−1.
Also, in a violet-light-emitting semiconductor laser device with an oscillation wavelength of about 400 nm, which has now been developed, a stacked structure made up of silicon dioxide and titanium oxide films is used as an end facet reflective film. For example, a semiconductor laser device of aluminum indium gallium nitride (AlInGaN) in which a stack of silicon dioxide and titanium oxide films is used as the end facet reflective film was reported in Jpn. J. Appl. Phys. Vol. 38 (1999) pp. L184-L186. The absorption coefficient of titanium oxide to light with a wavelength of 400 nm is 2400 cm−1.
Recently, a semiconductor laser device for an optical disk apparatus has to increase its output power to speed up a recording operation on an optical disk and to decrease its wavelength to increase the recording density.
However, there is a problem that neither the known end facet reflective film as a stack of silicon dioxide and amorphous silicon films for the infrared or red-light-emitting semiconductor laser device with an oscillation wavelength of about 780 nm nor the known end facet reflective film as a stack of silicon dioxide and titanium oxide films for the red-light-emitting semiconductor laser device with an oscillation wavelength of about 650 nm can meet the demand of increasing the output power of laser devices.
Further, there is another problem that the end facet reflective film as a stack of silicon dioxide and titanium oxide films for the violet-light-emitting semiconductor laser device with an oscillation wavelength of about 400 nm cannot meet the demand of decreasing the wavelength of laser devices.
This is because the light, emitted from these semiconductor laser device, is absorbed into the high-refractive-index film at an insufficiently small coefficient. Accordingly, if any of these laser devices increases its output power, the temperature rises markedly due to the increase in the amount of light absorbed into the high-refractive-index film. As a result, the crystallinity of the semiconductor laser devices deteriorates especially in part of the active region near the resonant cavity end facet thereof.
In the same way, if a laser device should have an oscillation wavelength as short as 400 nm or less, it is difficult to operate the device properly with a known end facet reflective film as a stacked structure including silicon dioxide. This is because the absorption coefficient of titanium oxide increases greatly at those short wavelengths.